Method for initiating cycle operation



Dec. 24, 1963 F. G. clAPl-:TTA ETAL. 3,115,456

METHOD FOR INITIATING CYCLE OPERATION Filed July 6, 1959 m E umm NrUnited States Patent O M 3,115,456 METHOD FOR INITIAHNG CYCLE @PERATIGNFrank G. Ciapetta, Silver Spring, Md., and Harry L.

Coonradt, Woodbury, and William Tl. Garw'ood, Haddoniield, NJ.,assignors to Soeony Mobil @il Company,

Inc., a corporation of New York Filed lilly 6, 1959, Ser. No. 825,133 6Claims. (tCl. 20S-111) This application is a continuation-in-part ofcopending application Serial Number 551,701, led December 7, 1955, nowabandoned.

This invention relates to catalytic cracking. 1t is more particularlyconcerned with the initiation of cycle operation of a cracking processthat is carried out in the presence of hydrogen.

In copending application Serial Number 813,247, filed May 14, 1959,which is a continuation-in-party of Serial Number 491,256, filed March1, 1955, now abandoned, a cracking process is described that involvescontacting a blend of virgin stock and cycle stock with a platinum orpalladium-containing catalyst in the presence of h"- drogen atsuperatmospheric pressures. This operation, as compared with aonce-through operation with the virgin stock alone, offers manyadvantages, including lower temperature, lower dry gas yield, lowerlight naphtha yield, greater heavy naphtha yield, and a fuel oil havinga higher diesel index. These advantages are especially pronounced in thecase of heavy charge stocks, such as heavy gas oils, vacuum gas oils,topped crude, etc., as these charge stocks require relatively severecracking conditions in a once-through operation. Recycle operationresults in greater catalyst stability during long runs. Thus, in aonce-through operation using heavy gas oil, the catalyst loses activityat the rate of about 3 F. per day. On the other hand, in cycle operationthis loss in activity is reduced to less than 1 P. per day.

Ordinarily, sufcient cycle stock derived from the heavy stocks will notbe available to initiate a recycle operation. To be sure, cycle stockcan be obtained by cracking the heavy charge stock in a once-throughoperation until suflicient is produced. This manner of operation,however, is disadvantageous to over-all refinery operation, because therelatively large yields of dry gas and light naphtha decrease the totaluseful output.

1t has now been found that cycle operation for cracking heavy stocks canbe initiated by a method that is simple and economical. 1t has beendiscovered that the cycle operation of a cracking process carried out inthe presence of a platinumor palladium-containing catalyst and ofhydrogen can be initiated by rst cracking a blend of the heavy chargestock with a light gas oil until subficient cycle stock is available,and then discontinuing the use of light gas oil and carrying out thedesired recycle operation.

Accordingly, it is an object of this invention to provide a method forinitiating a recycle cracking operation. Another object is to provide amethod for initiating a process that involves cracking a mixture of aheavy charge stock and cycle stock. A specific object is to provide amethod for initiating a process that involves cracking a mixture of aheavy charge stock and cycle stock in contact with a platinumorpalladium-containing catalyst and in the presence of hydrogen. Anotherspecific object is to provide a method for obtaining suflicient cyclestock to commence the cycle cracking of a heavy charge stock that issimple and economical, and which does not involve oncethrough crackingof the heavy charge stock.

Other objects and advantages of the process of this invention willbecome apparent to those skilled in the art from the following detaileddescription considered in conjunction with the drawings, wherein3,115,456 Patented Dec. 24, 1963 FGURE 1 presents a diagrammaticrepresentation of the process of this invention;

FIGURE 2 presents the graphic relationship between the volume percentconversion into products boiling at temperatures lower than about 390 F.and the temperature for cracking a typical heavy gas oil, a typicalmixture of heavy gas oil with light gas oil, a typical mixture of heavygas oil with cycle stock obtained by cracking a mixture of heavy gas oilwith light gas oil, and a typical mixture of heavy gas oil with cyclestock obtained by cracking the heavy gas oil.

The present invention provides, in a recycle cracking process wherein amixture of a heavy gas oil with cycle stock obtained from cracking saidheavy gas oil and in which the volumetric ratio of said cycle stock tosaid heavy gas oil varies between about 0.1:1, respectively, and about10:1, respectively, is contacted with a catalyst comprising betweenabout 0.05 percent and about 20 percent, y weight of the catalyst, of atleast one metal of the platinum and palladium series deposited upon asynthetic composite of the solid oxides of at least two elements ofgroups HA, HEB, and 1V of the periodic arrange ment of the elements,having an activity index of at least 25, in the presence of hydrogen inamounts, expressed in molar ratio of hydrogen to hydrocarbon charge,within the range about 2 to about 80, at pressures about 100 pounds persquare inch gauge, at a liquid hourly space velocity within the rangeabout 0.1 to about 10, and at a temperature above about 500 F., themethod of initiating said recycle cracking process that comprisesinitially contacting with said catalyst a mixture of a light gas oilwith said heavy gas oil in which the volumetric ratio of said light gasoil to said heavy gas oil is within the range about 10:1, respectively,and about 0.1:1, respectively, in the presence of hydrogen and undersubstantially the same conditions of hydrogen to hydrocarbon molarratio, pressure, space velocity, and temperature that are employed insaid recycle cracking process, continuing to charge said mixture for aperiod of time to produce sufficient cycle stock for said recyclecracking process, and subsequently replacing said light gas oil in saidmixture of light gas oil and heavy gas oil with said cycle stock.

A method of operating the process of this invention is illustrated inFIGURE l. A light charge stock is introduced through a pipe 10 andpumped by means of a metering pump 11 through pipes 12 and 13 into afurnace, or other suitable heating means, 14. The heavy charge stock tobe cracked is introduced through a pipe 1S and is pumped by means of ametering pump 16 through pipes 17 and 13 and introduced into the furnace14 in admixture with the light charge stock.

The mixture of light and heavy charge stock is heated in the furnace 1dto the desired conversion temperature. The thus-heated mixture is passedthrough pipes 18 and 19 into a reactor' 20.

Hydrogen gas, or a gas rich in hydrogen, is introduced through a pipe 21and compressed by means of a compressor Z2 to the desired reactionpressure. The compressed hydrogen is then passed through a pipe 23 intoa heat exchanger 24 wherein it is heated to reaction temperature. Theheated hydrogen is then passed through pipe 19 in admixture with thehydrocarbon charge into the reactor 20. Alternatively, the mixture ofhydrogen and charge stocks can be heated together in a common heater.

The reactor 2l) contains a static bed of a platinum 0r palladium seriesmetal catalyst. Suitable catalysts are described in copendingapplication Serial Number 418,166, filed March 23, 1954. The mixture ofhydrocarbons and hydrogen gas are contacted with the catalyst at aliquid hourly space velocity and temperature sailicient to effect thedesired amount of conversion. In order to produce sufficient cycle stock(products boiling at temperatures higher than about 390 F.), it isgenerally preferred to operate the process so that there is effected nomore than 50 volume percent conversion into products boiling attemperatures lower than about 390 F. In general, the amount ofconversion into products boiling at temperatures lower than about 390 F.will be between about and about 50 volume percent. The total effluentfrom the reactor is removed through a pipe 2S and passed through a heatexchanger 26 wherein the products are cooled to temperatures at whichhydrogen can be separated. The thus-cooled product is passed through apipe 27 into a high pressure gas separator 28.

In the gas separator 28, a gas rich in hydrogen is separated from theliquid product. The hydrogen-containing gas is removed from the highpressure separator 28 through a pipe 29 and recycled to the process viapipe 23. In pipe 23 the recycle hydrogen gas is admixed with sufficientfresh hydrogen to maintain the desired hydrogen to oil molar ratio inthe process.

The liquid product from the separator 28 is removed through a pipe 30and passed through a depressuring zone 31 and a pipe 32 into afractionator 33. In the fractionator 33 the reaction products areseparated into suitable fractions. Ordinarily, the dry gas is removedthrough a pipe 34. Butanes are removed through a pipe 35 and light andheavy naphtha fractions are removed through pipes 36 and 37,respectively. The product boiling at temperatures higher than about 390F. is removed from the fractionator 33 through a pipe 38 and passed intoa suitable storage vessel 39.

When sufficient cycle stock has been obtained in the storage vessel 39to initiate recycle operation with a heavy charge stock alone, cyclestock is removed from the storage vessel through a pipe 40. Then, thecycle stock is pumped through a metering pump 41 through a pipe 42 intopipe 13 in an amount sufficient to effect the desired ratio of recycleto heavy charge stock. At the same time, the flow of light charge stockis discontinued. This operation is effected by suitable manipulation ofvalves 43 and 44. From this point on, the process then operates as astraight recycle process.

In an alternative method of operating the process, the storage vessel 39can be eliminated. Then, the recycle stock as it becomes available ispumped directly into pipe 13. As more and more cycle stock becomesavailable, the amount of light charge stock is reduced accordingly bysuitable control of valve 44 and of metering pump 11. When suiiicientcycle stock is being produced to maintain the recycle operation, theflow of light charge stock is discontinued entirely and the straightrecycle process continues.

Throughout the specification and the claims, the term conversion isintended to be a generic term for the amount of products boiling attemperatures lower than about 390 F. (10U-recycle), of gasoline, or offuel oil obtained in the process. It is expressed in terms of the volumepercent of the initial charge which is transformed in the process. Theamount of product boiling at temperatures lower than about 390 F. isobtained by subtracting the volume percent of cycle stock from 100percent, i.e., from the initial volume of the charge. The expression(100-recycle) is an abbreviation for 100 percent minus the volumepercent recycle. Dry gas refers to the methane, ethane, propane, andethylene and propylene produced in a cracking process, expressed interms of Weight percent of the initial charge. Light naphtha is theproduct that boils between about 125 F. and about 170 F. The heavynaphtha is the product that boils between about 170 F. and about 390 F.The diesel index of the fuel oil is a function of the A.P.I. gravity andthe aniline number, as defined by Becker et al. in the S.A.E. Journal(Transactions), vol. 35, No. 4, p. 377. The cracking activity of acarrier is expressed in terms of the percent, by volume, of a standardhydrocarbon charge which is cracked, under specific operatingconditions, in the Cat. A test. This test is described by Alexander andShimp in National Petroleum News, 36, page R-537 (August 2, 1944). Theunit for rating the cracking activity of a material is called theactivity index (Al.

Thg catalysts utilizable herein are those described in U.S. 2,945,806,issued July 19, 1960. Briefly, these catalysts comprise between about0.05 percent, by weight, and about 20 percent, by weight of the finalcatalyst, preferably between about 0.1 percent and about 5 percent, byweight, of the metals of the platinum and palladium series, i.e., thosehaving atomic numbers of 44-46, inclusive, 76-78, inclusive, supportedupon synthetic composites of two or more refractory oxides. The carrieris a synthetic composite of two or more solid oxides of the elements ofgroups IIA, 111B and IVA and B of the periodic arrangement of elements[1. Chem. Ed., 16, 409 (1939)]. These synthetic composites of refractoryoxides must have an activity index of at least about 25. They can alsocontain halogens and other materials which are known in the art aspromoters for cracking catalysts, or small amounts of alkali metals thatare added for the purpose of controlling the activity index of thecarrier. Non-limiting examples of the composites contemplated hereininclude silica-alumina, silica-zirconia, silica-alumina-zirconia,silica-alumina-thoria, alumina-boria, silicamagnesia,silica-alumina-magnesia, silica-alumina-uorine, and the like. Thepreferred support is a synthetic composite of silica and aluminacontaining between about 1 percent, by weight, and about 90 percent, byweight, of alumina. These synthetic composites of two or more refractoryoxides can be made by any of the usual methods known to those skilled inthe art of catalyst manufacture. Examples of methods of preparing themare set forth in U.S. 2,945,806, issued July 19, 1960.

The following example illustrates a method of preparing aplatinum-containing catalyst utilizable in the process of thisinvention:

EXAMPLE l A synthetic silica-alumina carrier or support containing l0percent by weight alumina was prepared by mixing an aqueous solution ofsodium silicate (containing 158 g. per liter of silica) with an equalamount of an aqueous acid solution of aluminum sulfate containing 39.4g. Al2(SO4)3 and 28.6 g. concentrated H2804 per liter. This mixture ofsolutions was dropped through a column of oil, wherein gelation of thehydrogel was effected in bead form. The bead hydrogel was soaked in hotwater (about F.) for about 3 hours. The sodium in the hydrogel was thenremoved by exchanging the gel with an aqueous solution of aluminumsulfate [1.5% A12(SO4)3 by weight] containing a small amount (0.2percent by weight) of ammonium sulfate. The thus-exchanged hydrogel beadwas water-washed. Then, it was dried in superheated steam (about 280-340F.) for about 3 hours and, finally, calcined at 1300 F. under a lowpartial pressure of steam for about 10 hours.

The silica-alumina beads were then crushed to pass through a l4-meshscreen and the material retained on a 25-mesh screen (U.S. StandardScreen Series) was used for catalyst preparation. Portions of thecrushed, calcined carrier were then barely covered with aqueoussolutions of chloroplatinic acid, of concentrations sufiicient toproduce the desired amount of metal in the finished catalyst. The excesssolution was removed by centrifuging. The thus-impregnated carrier wasthen dried at 230 F. for 24 hours. The catalyst was treated withhydrogen for 2 hours at 450 F. Then, it was activated in hydrogen for 2hours at 900 F. before it was used. The catalyst thus-prepared contained0.47 percent platinum, by weight of the catalyst. Its silica-aluminacarrier had an activity index of 46.

The light charge stock that is charged to the process of this inventionis a hydrocarbon fraction that has an initial boiling point of about390400 F. and an end boiling point of about 600-650 F. The heavy chargestock 1s a hydrocarbon fraction that boils within the range varyingbetween about 400 F. and about 1200 F. and higher. This material can bea heavy gas oil, a heavy fraction obtained by destructive hydrogenationof coals, tars, pitches, etc., a vacuum gas oil, mixtures of medium andheavy gas oils, topped deasphalted crudes, and the like. Both the lightand heavy charge materials can be gas oils, refractory cycle stocks fromconventional cracking, hydrocarbon fractions obtained by destructivedistillation of coal, tars, pitches, asphalts, etc.

The light charge stock that is admixed with the heavy charge stock canbe from the same source as the heavy charge stock, or it can be from adifferent source. The relative amounts of light and heavy stocks areadjusted so that the reaction conditions for cracking the mixture willsubstantially be the same as the reaction conditions ultimately to beused in cracking the heavy charge stock in a recycle operation.Generally, the volume ratio of light charge stock to heavy charge stockwill be within the range about :1, respectively, to `about 0.1:1,respectively, preferalbly between about 5:1, respectively, and 1:1,respectively.

The presence of even relatively small amounts of nitrogen compounds inthe charge stock causes the degree of conversion that can be attained atany given temperature to decrease. Thus, the higher the nitrogen contentthe greater will be the temperature needed to effect a given amount ofconversion. Since higher temperatures are usually associated with higherdry gas production, there will usually be a greater quantity of dry gasproduced when converting high nitrogen stocks to the same degree aslower nitrogen stocks.

It is, therefore, preferred for optimum operations that the crackingcharge stocks contain less than about 0.1 percent nitrogen, by weight,and still more preferably less than about 0.08 percent, by weight.

If desired, the nitrogen content of a charge stock may be reduced to anamount less than about 0.1 weight percent, and, preferably, below about0.08 weight percent, before charging to the process of this invention byany of the methods well-known in the art, such as, for example, acidtreatment, propane deasphalting, and hydrogenolysis under very highpressure, in contact with catalysts such as molybdenum or tungstenoxide, nickel sulfide, tungsten sulfide, cobalt molybdate, cobalttungstate, etc.

ln order to obtain the best results in the recycle cracking process thatis initiated by the method of this invention, the ratio of cycle stockto virgin stock (charge stock that has not been subjected to the processof this invention or to processes described in the aforementionedcopending applications) in the charge ordinarily should be at leastabout 0.l:1. The ratio of cycle stock to virgin stock can be `as high as:1 or higher. In practice, however, it is preferred to use cycle stockto virgin stock, volume ratios within the range about 0.1 :1 to about10:1 and higher and more preferably within the range about 1:1 to about5:1.

As has been mentioned hereinbefore the initiation process of thisinvention is carried out under substantially the same conditions as thefinal cracking process. Thus, conditions of hydrogen pressure,hydrogen-tobil molar ratio, and space velocity will usually be the same.Throughout the initiation process and the ultimate cracking process, thetemperature might be somewhat different. However, :a Volume ratio oflight `charge stock to heavy charge stock is selected so that the curvedefining the relationship between the temperature and the volume percentconversion into products boiling at temperatures lower than about 390 E.will be substantially identical with the curve defining thisrelationship in the ultimate cycle cracking process.

ln so far as is known, there is nothing critical about the reactiontemperature at which the process of this invention is carried out, andit is believed operable at any temperatures, conventional in the art, atwhich hydrocracking, with a net consumption of hydrogen by the chargestock, will occur. Of course, the reaction temperature should notgenerally exceed the minimum temperature at which the charge isconverted completely into products boiling below 390 F. or some otherend point that is more desirable. For the reaction to proceed at anacceptable rate, it is usually necessary that the reaction temperatureexceed about 500 F. Preferably, the reaction temperature should bewithin the range about 500 F. to about 825 F., and still more preferablywithin the range 650 F. to 825 F.

Likewise, in so far as is known, there is nothing critical about thereaction pressure employed in this invention and it will operate at anyhydrogen pressures, conventional in the art, which effect hydrocrackingwith a net consumption of hydrogen by the charge stocks. Very highpressures, however, make it necessary to employ very expensive,heavy-walled reactors. It is, therefore, preferable that the hydrogenpressure be within the range to 2500 pounds per square inch gauge(p.s.i.g.). The liquid hourly space Velocity should be within the rangeabout 0.1 to about 10, preferably between about 0.1 and about 4 volumesof reactant (as 60 E, liquid) per Volume of catalyst per hour` The molarratio of hydrogen to hydrocarbon should be within the range about 2 toabout 80, preferably between about 5 and about 50.

The cracking process can be carried out using conventional apparatus andschemes for effecting recycle operation in catalytic cracking. As thecatalyst remains active over long periods of time before it must beregenerated, the operation is advantageously carried out using a fixedbed of catalyst. Other techniques, however, can be used, such as themoving bed technique or the fluid bed technique.

EXAMPLE 2 The charge stock used in this run was a mixture containing57.7 volurne percent of light gas oil and 42.3 volume percent heavy gasoil distilled from Kuwait crude. Pertinent properties of these Kuwaitgas oils and of this mixture are as follows:

Light Heavy Gas Gas Gas Oil Oil Oil Blend A.P.l.. Gravity 39.1 27.6 33.8 Dlstillation, Vacuum Assay l;

LBJ? 418 5% 648 467 506 724 590 814 787 634 787 0. 91 2. 06 1. 51

l Distillation data for light gas oil are A.S.T.M. distillation.

The blend of light and heavy gas oil was subjected to cracking in thepresence of hydrogen and of the catalyst described in Example 1, afterthe latter had reached equilibrium, i.e., had been in continuousoperation for more than 5 days. The hydrogen pressure used was 1000p.s.i.g., the liquid hourly space velocity was 0.5 and the molar ratioof hydrogen-to-oil was 40. The cracking was carried out at a temperatureof about 730 F. There was effected 56.7 volume percent conversion intoproducts boiling below about 390 F. The gaseous products were separatedfrom the liquid products. Then, the liquid products were subjected tofractionation. The pertinent results of this run are set forth in TableI.

EXAMPLE 3 The charge used in this example was a mixture of 1 Volume ofcycle stock produced in the run described in Example 2 and 1 volume ofthe heavy Kuwait gas oil that was defined in Example 2. The cycle stockand the blend of cycle stock and heavy gas oil had the followingproperties:

Cycle Blend Stock A.Pl. Gravity 43.6 35.3 Distillatiou, Vaeum Assay 1;

E Sulfur, Weight Percent 1 Distillation data for the cycle stock areA.S.T.M. distillation data.

This blend was subjected to cracking in the presence of hydrogen and ofthe catalyst described in Example 1J after the latter had reachedequilibrium. The hydrogen pressure used was 1000 p.s.i.g., the liquidhourly space velocity was 0.5 and the molar ratio of hydrogen-to-oil was40. The cracking was carried out at a temperature of about 700 F.whereby there was effected a conversion of about 40.5 volume percentinto products boiling lower than about 390 F. Separation between thegaseous products and the liquid products was effected and the stock andthe blend of cycle stock and heavy gas oil had the following properties:

Cycle Blend Stock A.P.I. Gravity 36.0 31. 7 Distillaton, Vacuum Assay 1:

I.B.P 420 1 Distillation data for the cycle stock are A.S.T.M.distillation data.

Table I Charge Mixture Mixture Mixture of Light of Cycle Cycle and StockHeavy Kuwait Gas (4) and Heavy and Oil Heavy Gas Oil Heavy Gas Oil GasOil Example No 2 3 4 4 4 5 730 700 743 793 785 710 1, 000 1,000 1, 0001, 000 1, 100 l, 000 0. 5 5 0. 5 0 5 0.5 Hz/oil molar ratio 40 40 40 4040 Results:

Conversion, Vol. Percent l.- 56. 7 40. 5 28.1 69. 2 52. 4 46.9 Dry Gas,Wt. Percent. 2.1 1. 9 2. 2 5.9 3.8 1. 8 Butanes, Vol. Percent 7. 6 5. 54.8 14.8 11. 6 8.8 Pentanes, Vol. Percent 7. 6 5.1 4. 3 11. 9 9.1 6. 9Lt. Naphtha Vol. Percent. 5.6 2.6 3.7 9.7 7.6 4.8 Hvy. Naphtlia, Vol.Percent 46. 2 34. 3 23. 6 42. 4 36. 3 37. 2 Fuel Oil, Vol. Percent. 43.3 59. 5 71. 9 30. 8 47. 6 53.1 Diesel Index of Fuel Oi 73. 7 73. 6 64. 471.0 58. 9 68. 8

1 Conversion into products boiling at temperatures lower than about 390F.

liquid product was fractionated. Pertinent results are set forth inTable I.

EXAMPLE 4 The charge stock used in this run was the heavy Kuwait gas oilthat was defined in Example 2. This gas oil alone was subjected tocracking in the presence of hydrogen and of the catalyst described inExample 1 after the latter had reached equillibrium. The hydrogenpressure used was 1000 p.s.i.g., the liquid hourly space velocity was0.5 and the molar ration of hydrogento-oil was 40. Cracking was carriedout in three runs. Each run was effected at a different temperature,namely about 743 F., about 793 F. and about 785 F. The products of eachrun were separated into gaseous and liquid products and the liquidproducts were fractionated. Pertinent results are set forth in Table I.

EXAMPLE 5 The charge stock used in this run was a mixture of 1 volume ofthe heavy Kuwait gas oil described in Example 2 and 1 volume of thecycle stock obtained by cracking the heavy Kuwait gas oil at atemperature of about 785 F., as described in Example 4. The cycle Therun described in Example 2, using the mixture of light and heavy gasoil, illustrates the initial stages of the initiating process of thisinvention. As described hereinbefore, this operation is continued untilsufficient cycle stock is obtained to use a cycle operation. At thattime, the charging of the light gas oil is discontinued (eithersuddenly, or gradually, as described hereinbefore) and the operation iscontinued using a charge composed of cycle stock and the heavy gas oil.This operation is illustrated in Example 3. Ultimately, of course, thecycle stock obtained by cracking the mixture of light and heavy gas oilsthat was used to initiate the cycle operation will become extinct. Atthat time the operation will involve a recycle operation wherein therecycle stock is obtained by cracking the heavy gas oil. This ultimatestage of the operation is illustrated by the run described in Example 5.

The curves set forth in FIGURE 2 are based upon the data in Table I.Curve A shows the graphic relationship between temperature and thevolume percent conversion of the charge into products boiling at temperatures lower than 390 F., when cracking under substantially identicalconditions, the mixture of light and heavy gas oils; the mixture ofheavy gas oil and cycle stock obtained from cracking the mixture oflight and heavy gas oil; and the mixture of heavy gas oil and cyclestock obtained from cracking the heavy gas oil. It will be noted thatthe volume percent conversiontemperature relationship for all theseoperations falls upon a common curve. Thus, for example, the blend of57.7 volume percent of light gas oil and 43.7 voiurne percent heavy gasoil cracks in the same temperatureconversion relationship as does theultimate recycle operation using the heavy gas oil plus cycle stocktherefrom in a 1:1 volume ratio. In other words, if the temperature ofthe ultimate operation, such as is described in Example were elevated to730 F. as used in the initiating run, there would be obtained aconversion of about 56 volume percent into products boiling atternperatures lower than about 390 F.

Curve B deiines the relationship between the temperature and the volumepercent conversion into products boiling at temperatures lower than:about 390 F. obtained when cracking the heavy Kuwait gas oil. Thiscurve is included for purposes of comparison. It will be noted that theinitiating process and the ultimate recycle operation described hereinare all eiected at temperatures substantially lower than those requiredfor cracking the heavy gas oil alone. It has been found that therelationship between the volume percent conversion and (l) the weightpercent dry gas, (2) the volume percent light naphtha and (3) the volumepercent heavy naptha will dene a common curve, as Iwas experienced inthe case of curve A in FIGURE 2. A comparison of these curves withcorresponding curves obtained by cracking the heavy gas oil alone showsthat there is achieved all the advantageous results describedhereinbefore, namely lower dry gas yield, lower light naphtha yield anda higher yield of heavy naphtha.

Another `advantage of this invention may be noted. With catalysts of thetype described herein there is an initial period of operation, when thecatalyst is fresh, in which the catalyst perfor-ms in a quite differentmanner from its typical performance after some aging has occurred. Useof a heavy gas oil charge during this period has been foundsubstantially less desirable than use of a light gas oil because theheavy gas oil has, on occasion, damaged specific catalyst functions. Theuse of a blend of light and heavy gas oils during the start-up period inthis invention insures that lthis damage does not occur.

Although the present invention has been described with preferredembodiments, it is to be understood that modifications and variationsmay be resorted to, without departing from the spirit and scope of thisinvention, as those skilled in the art will readily understand. Suchvariations and modiiications are considered to be within the purview andscope of the appended claims.

We claim:

l. In a recycle cracking process wherein a mixture of a heavyhydrocarbon charge stock with cycle stock boiling above about 390 F.obtained from hydrocracking said heavy hydrocarbon charge stock and inwhich the volumetric ratio of said cycle stock to said heavy hydrocarboncharge stock varies between about 1:1, respectively, and about 101:1,respectively, is contacted with a catalyst comprising between about 0.05percent and about percent, by weight of the catalyst, of at least onemetal of the platinum and palladium series of metals deposited upon asynthetic composite of the solid oxides of at least t-wo elements ofgroups IIA, IIIB and IV of the periodic arrangement of the elements,said composite having an activity index of at least 25, in the presenceof hydrogen in amounts, expressed in molar ratio of hydrogen tohydrocarbon charge, varying between about 2 and about S0, at pressuresvarying between about 100 pounds per square inch gauge and about 2500pounds per square inch gauge, at a liquid hourly space velocity varyingbetween rabout 0.1 and about 10, and at temperatures varying betweenabout 500 F. and about 825 F.; the

method of initiating said recycle cracking process that comprisesinitially contacting with said catalyst a mixture of a light hydrocarboncharge stock boiling within the range of about 390 F. to 650 F. withsaid heavy hydrocarbon charge stock, in which the volumetric ratio ofsaid light hydrocarbon charge stock to said heavy hydrocarbon chargestock varies between about 1:1, respectively, and about 10:1,respectively, in the presence of hydrogen, and under substantially thesame conditions of hydrogen to hydrocarbon molar ratio, pressure, spacevelocity and temperature that are employed in said recycle crackingprocess, continuing to charge said mixture for a period of time .toproduce suiiicient cycle stock for said recycle cracking process, andsubsequently replacing said light hydrocarbon charge stock in saidmixture of light hydrocarbon charge stock and heavy hydrocarbon chargestock with said cycle stock.

2. In a recycle cracking process wherein a mixture of a heavy gas oilboiling within the range varying between about 400 F. and about l200 F.with cycle stock boiling above about 390 F. obtained from hydrocrackingsaid heavy gas oil and in which the volumetric ratio of said cycle stockto said heavy gas oil varies between about 1:1, respectively, and about10:1, respectively, is contacted with a catalyst comprising betweenabout 0.05 percent and about 20 percent, by weight of the catalyst, ofat least one metal of the platinum and palladium series of metalsdeposited upon a synthetic composite of the solid oxides of at least twoelements of groups IIA, IIIB and 1V of the periodic arrangement of theelements, said composite having an activity index of at least 25, in thepresence of hydrogen in amounts, expressed in molar ratio of hydrogen tohydrocarbon charge, varying between about 2 and about 80, at pressuresvarying between about 100 pounds per square inch gauge and about 2500'pounds per square inch gauge, at a liquid hourly space velocity varyingbetween about 0.1 and about 10, and at temperatures varying betweenabout 500 F. and about 825 F., the method of initiating said recyclecracking process that comprises initially contacting with said catalysta mixture of a light gas oil boiling within the range varying betweenabout S-400 F, and about 600-650 F. with said heavy gas oil in which thevolumetric ratio of said light gas oil to said heavy gas oil variesbetween about 1:1, respectively, and about 10:1, respectively, in thepresence of hydrogen, and under substantially the same conditions ofhydrogen to hydrocarbon molar ratio, pressure, space velocity andtemperature that are employed in said recycle cracking process,continuing to charge said mixture for a period of time to producesuicient cycle stock for said recycle cracking process and subsequentlyreplacing said light gas oil in said mixture of light gas oil and heavygas oil with said cycle stock.

3. In a recycle cracking process wherein a mixture of a heavy gas oilboiling within the range varying between about 400 F. and about l200 F.with cycle stock boiling above about 390 F. obtained from hydrocrackingsaid heavy gas oil and in which the volumetric ratio 0f said cycle stockto said heavy gas oil varies between about 1:1, respectively, and about5:1, respectively, is contacted with a catalyst comprising between about0.1 percent and about 5 percent, by weight of the catalyst, of platinumdeposited upon a synthetic composite of silica and alumina, saidcomposite having an activity index of at least 25, in the presence ofhydrogen in amounts, expressed in molar ratio of hydrogen to hydrocarboncharge, varying between about 5 and and about 50, at pressures varyingbetween about 350 pounds per square inch gauge and about 2000 pounds persquare inch gauge, at a liquid hourly space velocity varying betweenabout 0.1 and about 4, and at temperatures varying between about 650 F.and about 825 F.; the method of initiating said recycle cracking processthat comprises initially contacting with said catalyst a mixture of alight gas oil boiling within the range varying between about 3904400 F.and about 600-650" F. with said heavy gas oil in which the volumetricratio of said light gas oil to said heavy gas oil varies between about1:1, respectively, and about 5: 1, respectively, in the presence ofhydrogen and under substantially the same conditions of hydrogen tohydrocarbon molar ratio, pressure, space velocity and temperature thatare employed in said recycle cracking process, continuing to charge saidmixture for a period of time to produce suicient cycle stock for saidrecycle cracking process, and subsequently replacing said light gas oilin said mixture of light gas oil and heavy gas oil with said cyclestock.

4. In a recycle cracking process wherein a mixture of a heavy Kuwait gasoil boiling within the range varying between about 650 F. 5 percentpoint in vacuum assay and about 815 F. 95 percent point vacuum assaywith cycle stock boiling above about 390 F. obtained from hydrocrackingsaid heavy gas oil and in which the volumetric ratio of said cycle stockto said heavy gas oil varies between about 1:1, respectively, and about5:1, respectively, is contacted with a catalyst comprising between about0.1 percent and about 5 percent, by weight of the catalyst, of platinumdeposited upon a synthetic composite of silica and alumina, saidcomposite having an activity index of at least 25, in the presence ofhydrogen in amounts, expressed in molar ratio of hydrogen to hydrocarboncharge, varying between about 5 and about 50, at pressures varyingbetween about 350 pounds per square inch gauge and about 2000 pounds persquare inch gauge, at a liquid hourly space velocity varying betweenabout 0.1 and about 4, and at temperatures varying between about 650 F.and about 825 F., the method of initiating said recycle cracking processthat comprises initially contacting With said catalyst a mixture of alight Kuwait gas oil boiling within the range varying between about 400F. and about 650 F. with said heavy gas oil in which the volumetricratio of said light gas oil to said heavy gas oil varies between about1:1, respectively, and about 5:1, respectively, in the presence ofhydrogen and under substantially the same conditions of hydrogen tohydrocarbon molar ratio, pressure, space velocity and temperature thatare employed in said recycle cracking process, continuing to charge saidmixture for a period of time to produce suicient cycle stock for saidrecycle cracking process, and subsequently replacing said light gas oilin said mixture of light gas oil and heavy gas oil with said cyclestock.

5. In a recycle hydrocracking process in which a mixture of a heavyhydrocarbon charge stock and a cycle stock boiling above about 390 F.obtained from hydrocracking said heavy hydrocarbon charge stock ishydrocracked by passing said mixture at hydrocracking reactionconditions over a catalyst comprising at least one metal of the platinumand palladium series of metals, in an amount within the range about 0.05to 20 percent, by weight of the catalyst, deposited on a syntheticcomposite of the solid oxides of at least two elements of groups IIA,IIIB and IV of the periodic arrangement of elements, said compositehaving an activity index of at least 25, the method of initiating saidrecycle cracking process which comprises: initially contacting saidcatalyst with a mixture of a light hydrocarbon charge stock boilingwithin the range about 390 F. to 650 F. and said heavy hy-V drocarboncharge stock, the volumetric ratio of said light hydrocarbon chargestock to said heavy hydrocarbon charge stock being within the rangeabout 0.1:1 to 10:1 and hydrocracking said mixture under hydrocrackingconditions which result in a net consumption of hydrogen by the mixture;continuing to charge said mixture until there is produced sufficientcycle stock to operate said process in the usual manner and thendiscontinuing the use of light hydrocarbon charge stock in said mixtureand replacing the light hydrocarbon charge stock with cycle stockproduced in the process.

6. The process of claim 5 in which, as the cycle stock is produced, itis returned to said catalyst and replaces an equivalent volume of lighthydrocarbon charge stock in said mixture until said light hydrocarboncharge stock is completely replaced with cycle stock.

References Cited in the file of this patent UNITED STATES PATENTS2,479,109 Haensel Aug. 16, 1949 2,799,626 Johnson et al. July 16, 19572,862,874 Boedeker et al. Dec. 2, 1958 2,885,346 Kearby et al May 5,1959 2,914,461 Ciapetta Nov. 24, 1959

3. IN A RECYCLE CRACKING PROCESS WHEREIN A MIXTURE OF A HEAVY GAS OILBOILING WITHIN THE RANGE VARYING BETWEEN ABOUT 400*F. AND ABOUT 1200*F.WITH CYCLE STOCK BOILING ABOVE ABOUT 390*F. OBTAINED FROM HYDROCRACKINGSAID HEAVY GAS OIL AND IN WHICH THE VOLUMETRIC RATIO OF SAID CYCLE STOCKTO SAID HEAVY GAS OIL VAIRES BETWEEN ABOUT 1:1, RESPECTIVELY, AND ABOUT5:1, RESPECTIVELY, IS CONTACTED WITH A CATALYST COMPRISING BETWEEN ABOUT0.1 PER CENT AND ABOUT 5 PERCENT, BY WEIGHT OF THE CATALYST, OF PLATINUMDEPOSITED UPON A SYNTHETIC COMPOITE OF SILICA AND ALUMINA, SAIDCOMPOSITE HAVING AN ACTIVITY INDES OF AT LEAST 25, IN THE PRESENCE OFHYDROGEN IN AMOUNTS, EXPRESSED IN MOLAR RATIO OF HYDROGEN TO HYDROCARBONCHARGE, VARYING BETWEEN ABOUT 5 AND AND ABOUT 50, AT PRESSURES VARYINGBETWEEN ABOUT 350 POUNDS PER SQUARE INCH GAUGE AND ABOUT 2000 POUNDS PERSQUARE INCH GAUGE, AT A LIQUID HOURLY SPACE VELOCITY VARYING BETWEENABOUT 0.1 AND ABOUT 4, AND AT TEMPERATURES VARYING BETWEEN ABOUT 650*F.AND ABOUT 825*F.; THE METHOD OF INITIATING SAID RECYCLE CRACKING PROCESSTHAT COMPRISES INITIALLY CONTACTING WITH SAID CATALYST A MIXTURE OF ALIGHT GAS OIL BOILING WITHIN THE RANGE VARYING BETWEEN ABOUT 390-400* F.AND ABOUT 600-650*F. WITH SAID HEAVY GAS OIL IN WHICH THE VOLUMETRICRATIO OF SAID LIGHT GAS OIL TO SAID HEAVY GAS OIL VARIES BETWEEN ABOUT1:1 RESPECTIVELY, AND ABOUT 5:1, RESPECTIVELY, IN THE PRESSENCE OFHYDROGEN AND UNDER SUBSTANTIALLY THE SAME CONDIDTIONS OF HYDROGEN TOHYDROCARBON MOLAR RATIO, PRESSURE, SPACE VELOCITY AND TEMPERATURE THATARE EMPLOYED IN SAID RECYCLE CRACKING PROCESS, CONTINUING TO A CHARDESAID MIXTURE FOR A PERIOD OF TIME TO PRODUCE SUFFICIENT CYCLE STOCK FORSAID RECYCLE CRACKING PROCESS, AND SUBSEQUENTLY REPLACING SAID LIGHT GASOIL IN SAID MIXTURE OF LIGHT GAS OIL WITH SAID CYCLE STOCK.